In a membrane filtration plant that uses filtration membranes such as ultrafiltration membranes or precise filtration membranes, a liquid to be separated is passed through a membrane module, and pressure is applied to the liquid from outside the membrane module. Desired filtration is then carried out under conditions at which a certain flux can be obtained, based mainly on the size of the pores in the membranes.
The nature of the liquid to be treated in a membrane filtration plant varies between membrane filtration plants, and it is often the case that various substances contained in the liquid cause so-called fouling such as clogging of the membranes so that the flux decreases rapidly or gradually. In a membrane filtration plant, physical cleaning such as air bubbling, or flushing is thus carried out repeatedly at relatively short time intervals to restore the membrane performance to some extent. Moreover, within a condition assuming chemical cleaning that thoroughly restores the membrane performance to be carried out, for example, once every six months, and within a range of possible operating conditions, a state under which operation can be carried out stably over the six months, by the next chemical cleaning, is determined, and out of such conditions, operation is generally carried out using conditions for which the efficiency is best.
Typical behavior of the operating pressure during stable operation under the condition of constant flow rate in a membrane filtration plant is shown in FIG. 10(a). In FIG. 10(a), the horizontal axis shows the operating time in days, and the vertical axis shows the operating pressure. An enlarged view of the portion enclosed by the circle in FIG. 10(a) is shown in FIG. 10(b). FIG. 10(b) shows the short-term pressure change associated with periodic cleaning. As can be seen from FIG. 10(a), the operating pressure rises rapidly at the start of operation. However, once the initial period has elapsed, a stable period begins and the operating pressure gradually rises with a constant gradient with the operating time. After the stable period, a final period begins and the operating pressure rises rapidly approaching the operating limit of a liquid feeding pump, whereupon chemical cleaning of the filtration membranes becomes necessary.
As the conditions of the operation of the membrane filtration plant, when assuming in advance that an operation time period is from this initial period to this final period and that the short-term cleaning is carried out, it is most efficient to operate at the maximum flux at which operation can be carried out stably at a constant flow rate over this time period. Therefore, when designing the membrane filtration plant, the maximum value of the flux in a stable state is estimated, taking the fixed short-term cleaning conditions and an operating time period between the chemical cleanings into consideration, and the design scale of the membrane filtration plant is determined accordingly.
However, the stable state flux during actual operation is affected by the type of substances contained in, the nature of particles in, and the concentration and so on of the liquid to be treated with pre-treatment included, and furthermore is considered to be affected in a complex way by various conditions such as the filtration membrane characteristics, interaction between substances contained in the liquid to be treated and the filtration membranes, the filtration membrane cleaning conditions, the operating conditions, and so on. Conventionally, due to such complex interactions, it has been considered to be completely impossible to estimate the stable state flux value in advance.
As an attempt to estimate this, there has been proposed, for example, a method known as the SDI (silt density index) measurement method in which the liquid to be treated is subjected to filtration for a fixed time at a constant pressure using a certain filtration filter, and it is attempted to determine the stable state flux value from the measured value of the flow rate at the time of starting the filtration and the time of ending the filtration. However, this method can only be used in a very narrow water quality range, and hence is not very practicable. Moreover, in Japanese Patent Application Laid-open No. 2001-327967 (Patent Document 1), there is described a method in which it is attempted to optimize a membrane filtration flux, a physical cleaning interval, a chemical cleaning timing, pre-treatment and so on from a function of measured values of a turbid matter amount and a soluble organic carbon amount, and the membrane filtration flux. However, in that invention, DOC, E260, and turbidity must be analyzed, which is complicated. Moreover, the cause of organic contamination is specified as being humic matter, the extent of contamination being calculated purely from the ratio between DOC and E260, and hence in the case that organic matter other than humic matter contributes to membrane contamination, the effect thereof cannot be properly evaluated.
Conventionally, when designing a new membrane filtration plant, it has thus generally been the case that membrane module(s) with one type or a plurality of types of candidate membrane(s) is/are used, and while using various combinations of pre-treatment and membrane module empirically or through trial and error, long-term operation for from a minimum of approximately one month to a maximum of approximately one year including seasonal variations is carried out in advance by actually passing the liquid to be treated through the membrane module, and it is tested through trial and error what is the maximum value of the flux that can be obtained stably. For example, in Non-Patent Document 1 (Advanced Aqua Clean Technology for 21st Century (ACT 21) New Development of City Water Membrane Filtration Technology, published by the Japan Water Research Center, December 2002, pages 200-204, 227-230, 257-271, 272-274, 277-279), various similar test results are reported, including a report of results of tests in which the long-term stability of a membrane water purification treatment system was investigated while testing various types of pre-treatment using an ultrafiltration (UF) membrane at Gifu Prefecture Yamanouchi Water Purification Plant.
Alternatively, in the case that such a long time cannot be taken for testing, it has been the case that empirical values for past membrane filtration plants for which the composition of the liquid to be treated is thought to be relatively similar are consulted, and thus the stable state flux value for the new membrane filtration plant is assumed empirically, and then a safety factor larger than usual is applied thereto so as to obtain the design value.